1. Field of the Invention
This invention relates to an image forming apparatus which forms an image by transferring a toner image that is formed on an image carrier such as a photosensitive drum to a record sheet held by a sheet carrier.
2. Description of the Related Art
There is a system in which a sheet carrier, around which a dielectric film is wrapped, holds a record sheet and a toner image is transferred to the record sheet by synchronously rotating the sheet carrier and an image carrier. This system has been known as one system for transferring the toner image formed on an image carrier, such as a photoreceptor drum, to a record sheet. In this system, the record sheet electrostatically holds the dielectric film by affording electrical charges to the dielectric film, which advantageously results in the conveyance of the record sheet and the separation of a latent image carrier from the record sheet in association with the transfer of the toner image.
For these reasons, this system is suitable for a color copier which is required to effect multiple transfer of toner images in a plurality of colors to the record sheet. In a conventional color copier, the sheet carrier holding the record sheet in an absorptive manner is repeatedly rotated in synchronism with the image carrier, so that color toner images, i.e., black, yellow, cyan, and magenta images formed in this order on the image carrier, are repeatedly transferred to the record sheet.
In the image forming apparatus that transfers a toner image in this way, sheet detection sensors are usually disposed at several points in the sheet carrier so as to be able to detect the record sheet held by the sheet carrier, as required. These sensors are provided in order to detect whether the record sheet is held at a predetermined location on the sheet carrier at predetermined timing or has become separated from that location. An error in absorption or separation of the record sheet is judged based on a detection signal from the sheet detection sensor. If such sheet conveyance error arises, an image forming operation will be immediately stopped.
A reflection photosensor made up of a light emitting portion and a light receiving portion in combination has conventionally been used for the sheet detection sensor. The photosensor is arranged in such a way as to judge whether or not the record sheet has passed an imaginary line connecting between the light emitting portion and the light receiving portion of the sheet detection sensor on the basis of a difference in the amount of light reflected from the sheet carrier itself and the amount of light reflected from the record sheet. FIG. 8 shows a detection signal of the sheet detection sensor which varies corresponding to the amount of reflected light received by the sensor. This plot shows that the detection signal output becomes larger as the amount of reflected light becomes smaller. The record sheet is specifically detected by checking whether or not the detection signal output is larger than a predetermined threshold value. When the white plain leading end of the record sheet has passed the imaginary line between the light receiving portion and the light emitting portion of the sheet detection sensor, the detection signal output varies so as to cross the threshold value, so that the record sheet is detected.
However, in the conventional image forming apparatus which detects the record sheet on the sheet carrier in the manner as previously mentioned, if the area ratio of a toner image transferred to the record sheet was considerably large, and if a margin between the leading end of the record sheet and the image was extremely small, it was impossible to accurately detect whether or not the leading end of the record sheet had passed the imaginary line between the light receiving portion and the light emitting portion of the sheet detection sensor. This was because a smaller amount of light reflected from the leading end of the record sheet led to a smaller difference between the amount of light reflected from the leading end of the sheet and the amount of light reflected from the sheet carrier, as shown in FIG. 9. Particularly in the case of a color copier, solid images such as photographs and pictures are frequently copied. If a margin between the leading end of the record sheet and the image is set smaller in order to enhance the appearance of a copied image, it becomes impossible to detect the record sheet the instant when several color toner images have been transferred.
Further, when an OHP film is used as the record sheet, the light emitted from the photosensor is reflected from the sheet carrier after having passed through the OHP film, which makes it impossible to detect the record sheet based on the detection signal from the photosensor. For this reason, an OHP film having its leading end painted in white has conventionally been used in the image forming apparatus with the aforementioned construction. However, since the area that was made opaque in white was small, it was difficult for the conventional image forming apparatus to accurately judge whether or not the sheet carrier absorbs the OHP film.
On the other hand, in the previously mentioned image forming apparatus in which the sheet carrier electrostatically absorbs the record sheet, it is necessary to remove residual electrical charges on the sheet carrier every time one cycle of the image forming operation has finished, in order to prevent the sheet carrier from erroneously absorbing the record sheet or to prevent a toner image from being erroneously transferred to the record sheet. To eliminate the residual electrical charges, corotrons for removing electrical charges are provided around the sheet carrier.
However, it was impossible to ascertain whether or not the corotron was perfectly functioning without the actual occurrence of erroneous holding of the record sheet or a failure of transfer of a toner image. In this way, it was impossible for the conventional image forming apparatus to prevent these problems.